Magnetic recording disks each include a nonmagnetic substrate on which at least a magnetic recording layer, a protective layer, and a lubricating layer, are arranged in that order. The lubricating layer has a function of reducing the impact of the magnetic recording disk against a magnetic head. The distance between the magnetic recording disk and the magnetic head becomes narrower and narrower with an increase of a recording capacity of a magnetic recording disk drive such as a HDD. Since a LUL (load/unload) system has been employed, the flying height of the magnetic head is reduced. The magnetic head needs to stably operate at an ultra-low flying height of 10 nm or less.
Under such conditions, the magnetic head may often instantaneously contact the magnetic recording disk rotating at high speed and therefore contact portions thereof are heated to high temperature (flash temperature) due to friction. Therefore, the lubricating layer needs to have high heat resistance so as not to be decomposed or vaporized due to heat under such high-temperature conditions.
The rotation of the magnetic recording disk at high speed causes the lubricating layer to migrate; hence, the thickness of the lubricating layer is likely to become nonuniform. In such a state, there is a problem in that fly stiction occurs when the magnetic head approaches from outside during an LUL operation. Fly stiction is a problem that the flying behavior and height of the magnetic head are changed during flight, causing irregular fluctuations in reproduction output. The magnetic head contacts the magnetic disk during flight in some cases. This can cause head crush. Therefore, the lubricating layer needs to be prevented from migrating.
In order to cope with such a need, a lubricant, such as FOMBLINE Z available from Solvay Solexis Inc., having high heat resistance and long-term stability is usually used to form lubricating layers for magnetic recording disks. The lubricant is purified such that impurities are removed from the composition or the molecular weight thereof is adjusted to an appropriate value. Weight-average molecular weight (Mw) or number-average molecular weight (Mn) has been usually used as a parameter for purification (see Patent Document 1).
[Patent Document 1] Japanese Unexamined Patent Application Publication (JP-A) No. 2004-319058